An ab initio/RRKM study of the reaction mechanism and product branching ratios of CH3OH+ and CH3OH++ dissociation

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Highlights

  • Ab initio/RRKM calculations reveal the mechanism of the CH3OH+ and CH3OH++ dissociation reactions.

  • CH2OH+ + H is calculated as the dominant product in CH3OH+ dissociation channels.

  • HCO+ + H3+ is calculated as the dominant product in CH3OH++ dissociation channels.

  • The fraction of roaming channel in the decomposition of CH3OH++ is 0.65 at Eint = 35.7 kcal/mol.

Abstract

Regarding CH3OH+ and CH3OH++, theoretical calculations have used a variety of methods to describe geometric structures and potential energy surfaces. Rice–Ramsperger–Kassel–Marcus (RRKM) theory has been applied to compute the rate constants and product branching ratios of various channels on potential energy surfaces. The dissociate ions produced from CH3OH+ include CH2+, HCOH+ and CH2OH+, whereas H+, H2+, H3+, COH+/HCO+, and CH3O+ fragments are generated from CH3OH++. By roaming, we imply that a neutral hydrogen molecule fragment explores relatively flat regions of the intrinsic reaction coordinate calculations from the minimum energy path.

Introduction

An intense femtosecond laser fields has an active area of research and it led to neutral hydrocarbon molecules can experience a variety of dynamical processes such as multiple ionization, dissociative ionization (DI), Coulomb explosion (CE), and chemical bond rearrangement [[1], [2], [3], [4], [5], [6], [7]]. For example, Yamanouchi and colleagues have measured the kinetic energy release for H2+ and H3+ ejected from the monocation and dication of methanol [1]. Coincidence momentum imaging was used to identify the hydrogen ejection processes for dication, CH3OH++ → CH3-nOH+ + Hn+ (n = 1–3), and angular distribution patterns were used to estimate lifetimes [2,3]. For the laser-pulse lengths of 7, 21, and 60 fs, the relative yields of CH3OH++ → CH3+ + OH+ and CH3OH++ → CH2OH2++ → CH2+ + H2O+ were 1:0.13, 1:0.17, and 1:0.5:0.9 [5]. A pump-probe coincidence momentum imaging method was used to study ultrafast hydrogen migration [6,7]. The 38-fs 800-nm pump produced methanol monocation, and a probe pulse delayed by 100–800 fs was used in coincident momentum imaging. The main possible product of methanol cation rearrangement is methyleneoxonium cation (CH2OH2+), a prototype distonic radical cation, and possible dissociation products include hydroxymethyl cation (CH2OH+), formaldehyde cation (CH2O+), hydroxymethylene cation (HCOH+), and isoformyl cation (COH+). As proposed by Hogness and Lunn [8], a bimolecular reaction involving neutral and singly ionized hydrogen molecules yields H3+, namely, H2 + H2+ → H3+ + H. The formation of H3+ starting from organic molecules is a unique chemical reaction as it requires cleavage and consecutive formation of three bonds. There has been evidence of hydrogen atom migration on ultrafast time scales as well as the production of H3+ from several organic molecules [9]. Ekanayake et al. [10,11] provided H2 roaming experiment in strong laser field combined with time-of-flight mass spectra and ab initio molecular dynamics simulations evidence that H3+ formation pathways tend to be unique because of the methanol cation structure. The elongation of C–H bonds and narrowing of the H–C–H angle of CH3 group are the primary initial steps in the formation of neutral roaming H2. Although extensive experimental studies [[1], [2], [3],10,11] have been published in these literatures, the role of various parent ions and their dissociation mechanisms is often difficult to understand in a general context.

The ground state potential energy surface for neutral methanol is well-known experimentally and has been the subject of numerous computational studies [[12], [13], [14]]. Calculations of the monocation potential surface by Bouma, Nobes, and Radom [15] showed that the lowest energy structure is H2COH2+ and not CH3OH+; this was quickly confirmed experimentally [16,17]. Pople, Radom, and colleagues [18] mapped the CH3OH+ potential energy surface at the G2 [19] level of theory and found it to agree closely with available experimental data. More detailed calculations by Radom and colleagues refined the energy difference between H2COH2+ and CH3OH+ [20] and showed that CH3OH had an eclipsed conformation [21,22]. However, Wu and colleagues [23] have observed in the time-of-flight mass spectrum of ionization and dissociation of methanol that the ion yields of fragment ion H3+ and H2O+ are slower than CHO+ and CH2+ respectively, which is consistent with their theoretical results according to which dissociation from the CH3OH+ to H3+ and H2O+ is more difficult than CHO+ and CH2+ respectively. Yamanouchi and colleagues revealed that a long lived neutral D2 moiety in CD3OH++ was the origin to the formation of HD2+ and D3+ (CD3OH++ → HD2+ + CDO+ and CD3OH++ → D3+ + COH+) by ab initio molecular dynamics simulation [24]. Ekanayake et al. [10,11] demonstrated that the H3+ formation from a series of doubly charged alcohols proceeded through a roaming neutral H2 moiety, and found that the yield of H3+ decreased as the carbon chain length increased. The fragmentation of doubly ionized methanol has been studied experimentally by Eland and colleagues using photoionization and photo-electron-photoion-coincidence detection [25,26]. Radom and colleagues explored the potential energy surface for the singlet dication and found H2COH2++ to be much stable than CH3OH++ [27,28].

According to the above studies, the mechanism of H3+ formation from CH3OH++ as a unimolecular decomposition process in which a long-lived neutral moiety of H2, formed within a dication molecule, abstracts a proton in the other moiety having the charge of +2. This suggested the possibility that the decomposition of the CH3OH++ might occur via a roaming radical mechanism [10,11]. Roaming dynamics are now widely recognized as an important pathway in unimolecular reactions [[29], [30], [31]] but their role in bimolecular reactions remains an open question. High-level ab initio calculations have been performed to unravel the CH3OH+ and CH3OH++ dissociation mechanisms and to provide accurate energies of intermediates and transition states along the studied pathways. Ab initio electronic structures, RRKM calculations have been conducted for rate constants of individual reaction steps, relative yields (branching ratios) of the products at various excess internal energies. Details learned of through study of the unimolecular photodissociation reactions presented in this letter enhance our understanding of H3+ reaction mechanism involving H2 roaming.

We assume here that the fragmentation occurs by the ionization followed by dissociation (ionization–dissociation) mechanism. This assumption is justified because, the rate of the electron motion is faster than that for the nuclear motion, if neutral dissociation take place, the molecule first has to absorb energy from the laser and then to either pre-dissociate or to decompose after internal conversion. The neutral fragments produced would be ionized afterwards. It is common that such processes take place on picosecond timescale or even slower, which is longer than the laser pulse duration of ∼100 fs, and hence this dissociation-ionization mechanism is rather unlikely.

The geometries of various transition states and fragmentation products on the potential energy surfaces (PESs) of CH3OH+ and CH3OH++ ions have been optimized at the B3LYP/6-311G(d,p) level [32,33] and single-point energies were refined by G3(MP2,CCSD)//B3LYP/6-311G(d,p) level [34,35] using the GAUSSIAN 09 package [36]. Vibrational frequencies were computed at the same B3LYP/6-311G(d,p) level to obtain zero point energies and to confirm that the structures were either minima or transition state (the number of imaginary frequencies NIMAG = 0 and 1 for the local minima and transition state, respectively). The intrinsic-reaction-coordinate (IRC) calculations at the same level of theory were carried out to track the minimum energy paths from the transition states to the corresponding minima. The computed frequencies and energies of the intermediates and transition states were utilized for microcanonical RRKM calculations of energy-dependent reaction rate constants using the formalism described in our previous work [37].

It is important to address the applicability of this statistical approach for describing the ionization-dissociation mechanism. The first issue concerns with the total amount of internal energy E accumulated by a molecule during the excitation-ionization process. In photodissociation molecular beam experiments, where photoexcitation to an electronically excited state is followed by relaxation to the ground state through internal conversion, the value of E is typically equal to the energy of the absorbed photon. Then, energy-dependent RRKM rate constants can be computed and product branching ratios derived from these rate constants are capable to closely describe experimental data measured under single-collision conditions, i.e., at the zero-pressure limit [[38], [39], [40]]. To estimate the internal energy E for multi-photon ionization-dissociation reactions, one needs to know the number of photons absorbed by a molecule and the translational energy of emitted electrons; otherwise, we can only consider a wide range of E. This problem was discussed by Sharifi et al. in relation to the dissociative ionization of methane in intense femtosecond laser field [41] and it was concluded that the parent ions are produced with a wide distribution of internal energies. This makes the microcanonical RRKM calculations a qualitative tool for understanding reaction mechanisms leading to various product ions rather than a quantitative method for accurately evaluating product branching ratios. Second, it is impossible to estimate the internal energy of daughter fragments produced by dissociation of the parent ion. The internal energy of secondary fragments should be lower than the energy of the parent ion because of energy redistribution among vibrational, translational, and rotational degrees of freedom of products. Thus, in the present letter we confine ourselves to a qualitative analysis of secondary reactions and do not calculate their product branching ratios due to the uncertainty in the internal energies and the complicated dissociation scheme of the parent ion.

Third, at high internal vibrational energies the RRKM calculated rate constants may exceed the applicability limit of RRKM theory, which normally is assumed to be 1014 s−1 - the typical rate of intramolecular vibrational redistribution (IVR). Therefore, the behavior of the system at high internal energies can deviate from the statistical (RRKM) behavior. Keeping these three points in mind, we emphasize that the results of our ab initio/RRKM calculations only provide a qualitative guide on the formation mechanisms of various products and their relative importance.

Section snippets

Dissociation mechanisms of CH3OH+ and CH3OH++

The multistep dissociation and rearrangement channels of CH3OH+ calculated at the G3(MP2,CCSD)//B3LYP/6-311(d,p) level are shown in Fig. 1. At the initial reaction step, one of the methyl group’s hydrogen atoms from CH3OH+ migrates to the O atom to form a CH2OH2+ intermediate through a transition state M+-TS1. The HCO angle of the dashed triangle in M+-TS1 measures 51.4°, and the dashed O–H bond is 1.217 Å. The breaking C–H bond length is 1.348 Å, so the bond is increased by 0.264 Å relative to

Conclusions

The PESs for the dissociation mechanism of CH3OH+ and CH3OH++ are explored using the G3(MP2,CCSD) level of theory. In this study of multiphoton resonance ionization followed by fragmentation, one can evaluate the range of internal energies available for fragmentation of the cations through absorb photon energies calculations. Calculations of PESs for various fragmentation channels and relative product yields at various available internal energies allow us to analyze the trends in branching

Author contributions

C.L. calculated the electronic structures. C.H.C. performed the RRKM calculations and analyzed the data. C.H.C, T.Z. and J.Z.H.Z. discussed the paper. C.H.C. and J.Z.H.Z. co-wrote the paper.

Declaration of interest statement

There are no conflicts of interest.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grants Nos. 91641116, 21433004 and 91753103), and the NYU Global Seed Grant. This work was supported by Laboratory and Equipment Management Office of ECNU. We also thank the ECNU Multifunctional Platform for Innovation (No. 001) for providing supercomputer time.

References (41)

  • H. Xu et al.

    Effect of laser parameters on ultrafast hydrogen migration in methanol studied by coincidence momentum imaging

    J. Phys. Chem. A

    (2012)
  • T.R. Hogness et al.

    The ionization of hydrogen by electron impact as interpreted by positive ray analysis

    Phys. Rev.

    (1925)
  • K. Hoshina et al.

    Efficient ejection of H3+ from hydrocarbon molecules induced by ultrashort intense laser fields

    J. Chem. Phys.

    (2008)
  • N. Ekanayake et al.

    Mechanisms and time-resolved dynamics for trihydrogen cation (H3+) formation from organic molecules in strong laser fields

    Sci. Rep.

    (2017)
  • N. Ekanayake et al.

    H2 roaming chemistry and the formation of H3+ from organic molecules in strong laser fields

    Nat. Commun.

    (2018)
  • L.B. Harding et al.

    Moller-Plesset study of the H4CO potential-energy surface

    J. Phys. Chem.

    (1980)
  • L.B. Harding et al.
  • H.G. Yu et al.

    MRCI calculations of the lowest potential energy surface for CH3OH and direct ab initio dynamics simulations of the O(1D) + CH4 reaction

    J. Phys. Chem. A

    (2004)
  • W.J. Bouma et al.

    The methylenoxonium radical cation (CH2OH2+) – a surprisingly stable isomer of the methanol radical cation

    J. Am. Chem. Soc.

    (1982)
  • W.J. Bouma et al.

    Experimental – evidence for the existence of a stable isomer of CH3OH+ – the methylenonium radical cation, CH2OH2+

    J. Am. Chem. Soc.

    (1982)
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